Methane is the second most prevalent greenhouse gas emitted in the United States. In general, by weight, methane is also a more potent greenhouse gas than other greenhouse gases, such as carbon dioxide. Methane is generated through both natural and human activities, including sources such as fossil fuel extraction, wetlands emissions, leakage from natural gas systems, waste and landfill systems, and livestock waste. Due to human activities, methane concentrations have increased largely during the 20th century and are now more than double pre-industrial levels.